With the increasing popularity of electric vehicles, wireless or contactless induction charging is becoming of interest. In an induction charging system, a primary coil operating at a high frequency, creates an alternating magnetic field that couples a secondary coil, which produces an alternating current that can be subsequently used by on-board electronics of the electric vehicle to charge its batteries. Power is delivered from the primary coil to the secondary coil over a relatively large air gap via the magnetic coupling. The charger can operate at a high frequency (typically from 30 kHz to 140 kHz) to deliver the required power with moderate sized equipment. Resonant circuits are normally used in the primary and secondary coils to enhance power transfer, while minimizing the required voltage and current from the power supply to the primary coil.
In an example magnetic coupling structure, the primary and secondary coils may have substantially the same dimensions. In electric vehicle charging, the secondary coil can be attached to the underside of the electric vehicle, while the primary coil can lie flat on the floor of a garage, for example, or it can be buried beneath pavement in a parking lot or in the ground. When the electric vehicle has stopped and positions its secondary coil over the primary coil, the primary and secondary coils occupy substantially parallel planes separated by an air gap of, for example, a few centimeters. Electric power can then be transferred across the air gap via the magnetic coupling from the primary coil to the secondary coil on the vehicle.